Add controller tests for switch_locale

This way it'll be easier to change it while checking we haven't broken
existing behavior.

While writing the tests, I noticed we were sometimes storing a symbol in
the session while sometimes we were storing a string. So we're adding a
`to_s` call so we always store a string in the session.

app/controllers/application_controller.rb

@@ -46,7 +46,7 @@ class ApplicationController < ActionController::Base
         current_user.update(locale: locale)
       end
 
-      session[:locale] = locale
+      session[:locale] = locale.to_s
       I18n.with_locale(locale, &action)
     end